1. Field of the Invention
The present invention relates to an image forming apparatus for dividing image data to printprocess the latter, and particularly to an image forming apparatus for controlling a supply of divided image data to print means.
2. Description of the Related Art
In an image forming apparatus such as an electronic photographic copier and a printer, an image formed on the basis of image data is printed on a sheet of paper by an electronic photographic process comprising a transfer process, a developing process, and a fixing process. Since a process speed of the electronic photographic process is constant, parameters of the processes constituting the electronic photographic process are designed on the basis of a predetermined process speed. For this reason, if the process speed is made variable, all the parameters of the processes concerned have to be made variable. This is not realistic.
As described above, in the image forming apparatus, the process speed is constant. Therefore, it is necessary to supply image data to print means (print engine) for forming an image from image data to print it on a sheet of paper at a constant speed according to the process speed. For example, in the case where the supply speed of image data to the print means is slower than the process speed, supply timing of paper to the print means is deviated from supply timing of image data, and as a result, a white sheet of paper not printed with an image is ejected or a disturbed image is printed on a sheet of paper halfway.
In the image forming apparatus, it is implemented that in communicating image data or in storing image data in memory means, image data is coded in order to reduce a quantity of data, and in printprocessing it, the image data is decoded. As described above, in the case where the image data is supplied in a coded state, the supply speed of image data to the print means depends on an original, since generally a speed for the coding process depends on an original, whereby matching of the supply timing of image data to the print means to the supply timing of a sheet of paper cannot be assured.
In the case where a coding system is a non-reversible coding system, control for a quantity of codes is made so that a constant quantity of codes is obtained per page whereby a decoding speed can be made constant. However, generally, there exist properties such that in the case where a certain image is non-reversible coded, the smaller the quantity of codes the larger the deterioration of image quality, and an image which is hard to be coded (for example, an image constituted at random noise) increases in deterioration of image quality in case of coding with the same quantity of codes as compared with a normal image (for example, a natural image such as a figure and a landscape). For this reason, in the case where a variety of images are coded with a constant quantity of codes by the control of a quantity of codes, the deterioration of image quality increases depending on images, sometimes failing to maintain an image which can withstand a practical use.
Accordingly, in the case where image data is coded and supplied, it is difficult to make the decoding speed constant, if an image is intended to be assured, making it difficult to supply image data to print means at a speed corresponding to the process speed.
In the case where it is difficult to make the decoding speed constant as described above, decoded image data is first retained in temporary memory means such as a buffer memory and the image data is supplied from the temporary memory means to the print means. Then, the supply of image data to the print means can be made at the speed corresponding to the process speed to prevent the situation such as ejection of white (unprinted) paper or printing of a disturbed image from occurring.
Therefore, in a conventional printing apparatus, a page memory (a semiconductor memory) capable of storing image data for one page portion or more is provided so that image data for one page portion or two-page portion are temporarily stored whereby image data are supplied to the print means according to the process speed.
For example, in the case where a page memory for one page portion is provided, as shown in FIG. 18A, image data for one page portion are stored in the page memory, after which a sheet of paper is fed from a paper feeder to the print means, and the print means prints images for one page portion on a sheet of paper fed on the basis of the image data stored in the page memory. The operation of the storage of image data, the feed of a sheet of paper and the printing of images is repeatedly implemented per page.
In the case where a page memory for a two-page portion is provided, as shown in FIG. 18B, the processing, in which while the printing process being is carried out on the basis of image data for one page portion stored in the page memory, image data for the next one page portion is stored in the page memory, is repeatedly implemented, thus enabling printing of a plurality of pages continuously.
In the case where the page memory for one page portion alone is provided as described above, the storage of image data and printing are sequentially carried out. Therefore, in the case where a plurality of sheets are continuously fed for printing, the through-put is lowered. On the other hand, in the case where the page memory for a two-page portion is provided, since the storage of image data and printing are carried out in parallel, the through-put is improved but a quantity of memory required for the page memory increases, resulting in an increase of cost.
The above-described things will apply to not only a monochromatic printer but also a color printer in which inks of color components, yellow (Y), magenta (M), cyan (C) and black (K), are placed upon one another and recorded.
In the color printer, as shown in FIG. 19, recorders (print means) exclusive-use for color components 100Y, 100M, 100C and 100K are provided, and these recorders 100Y, 100M, 100C and 100K are operated in pipeline adjusting to the process speed with respect to sheets of paper conveyed by a paper conveyor 101 to thereby enabling a through-put of color print close to that of monochromatic print.
In the pipeline color printer, the through-put is enhanced by subjecting sheets of paper to pipeline processing. Thus, when there is a vacancy in the pipeline (that is, sheets of paper cannot be passed continuously to the recorders for various colors), the through-put becomes lowered.
In the case where a plurality of the same originals are continuously printed, when image data for one page portion are prepared for the page memory for one page portion, sheets of paper are continuously fed to the pipeline, and the image data stored in the page memory are read plural times and supplied to the recorders 100Y, 100M, 100C and 100K plural times whereby images may be printed repeatedly on the plurality of sheets of paper fed.
On the other hand, in the case where a plurality of different originals are continuously printed, for example, a page memory for a portion of the number of sheets of originals is provided so that all the image data are prepared for the page memory, and after this, sheets of paper have to be fed to the pipeline. Because of this, when a page memory is constituted by a semiconductor memory, the greater part of printer cost is occupied by the cost of the semiconductor memory. Further, when a page memory is constituted by a magnetic disk, since a data transfer speed of the magnetic disk is greatly slower than a recording speed, a plurality of magnetic disks have to be operated in parallel. Thus, the greater part of printer cost is occupied by the cost of the magnetic disks.
That is, in the case where a plurality of different originals are continuously printed assuring the pipeline operation, a page memory (temporary memory means) for a depth portion, of a pipeline is necessary. The storage of image data to the page memory should have been completed before the supply of sheets of paper.
For example, in an example in which pages for different images are decoded with the time required for the recording by the recorders 100Y, 100M, 100C and 100K deviated, and the images are recorded sheet by sheet, the processing as shown in FIG. 20 is implemented. In this figure, the axis of abscissa indicates the time axis, in which below the timing at which a recording sheet of each page is processed the capacity of a page memory required at that time is shown by the memory capacity (1 plane) for one page and one color.
First, In the process for the first page, image data (four planes) for color components, yellow (Y), magenta (M), cyan (C) and black (K) are decoded and retained in the page memory. Then, paper feeding process for conveying recording sheets to the head position of a row of recorders is implemented, after which sheets of paper are conveyed between the recorders 100Y, 100M, 100C and 100K, and the recording processes are sequentially implemented on the basis of the image data of color components. As a result, after the time of 6 units, images for one page are recorded one upon another on sheets of paper for color components, and the data retaining capacity of the page memory with respect to one page is vacant "0."
In the state delayed by the time of one unit, the second page is processed in parallel, and in the state further delayed by the time of one unit, the third page is processed in parallel. In this manner, the capacity of the page memory which is necessary every completion of the recording process for one plane is reduced "1" by "1" and the pages are sequentially processed in parallel.
That is, in the above-described example, if the pipeline is filled after the time of 6 units from the start of printing and the page memory has the memory capacity for 18 planes at the maximum, different images can be recorded on the sheets of paper continuously fed.
In the case of the tandem type color printer as described above, data for a plurality of pages are retained in the page memory to enable the continuous feeding of recording sheets to carry out recording while preventing the through-put from being lowered.
Further, for example, in an image forming apparatus for handling image data of language described on the page, in carrying out the printing on sheets of paper, the data type of image data is sometimes processed to be converted, the things similar to the case of decoding process described above involve in the case where such conversion as described is carried out.
As described above, in the tandem type color printer, for example, in the case where a plurality of different images are printed, even if recording sheets of paper are intermittently fed, a page memory for a depth portion of a pipeline is necessary. In an example shown in FIG. 20, a page memory for 18 planes is necessary.
A page memory is constituted by a semiconductor memory which is high in access speed. Then, for example, in the case of image data having a resolution of JIS A4 format, 400 [dot/25.4 mm], and a gradation precision of each pixel 8 [bit/pixel], data quantity for one plane is about 16 [MByte]. Thus, a large capacity memory of about 288 [MByte] in total for 18 planes is necessary, resulting in a considerable increase in printer cost.
Also in a monochromatic printer, in order that data of different kinds of image pages are continuously printed, a memory having a capacity of about 32 [MByte] in total for two pages at the minimum is necessary, resulting in a considerable increase in printer cost caused by the memory cost.
Further, in the color printer, means for carrying out decoding and conversion process to supply image data to print means (recorders) are present independently every color component. Therefore, loads cannot be dispersed even in the case where the loads of supplying image data are one-sided every color component. Therefore, in the case where a load in respect of a color component is large, even if there is room in image data supply means for carrying out decoding and data conversion process as a whole, there sometimes occurred the things in which the print process by print means cannot be done.